pDC->SetWindowExt(1440, 1440);
pDC->SetViewportExt(pDC->GetDeviceCaps(LOGPIXELSX),
           -pDC->GetDeviceCaps(LOGPIXELSX));

pDC->SetWindowExt(1440, -1440);
pDC->SetViewportExt(pDC->GetDeviceCaps(LOGPIXELSX),
           pDC->GetDeviceCaps(LOGPIXELSX));
负号放在不同地方?逻辑坐标的方向是一样的吗?y都是向上递增?